Are these supposed to be intentionally humorous? When I first saw these, I started reading the rest of the article assuming it was meant to be a joke SCP.

Special Containment Procedures: Instances of SCP-XXXX are to be contained in a 10 x 10 meter habitat,

That's a pretty big habitat, which seems like it'd be troublesome for staff to keep clean and under observation. Why not make the habitat half of it, with some buffer areas?

floored with grass and soil, and containing several hollow overturned logs.

Why logs? Why not artificial structures appropriate to the things' apparent species? There are lots of things that can grow/live in/feed off of hollow logs, and I'm not sure if the Foundation wants to deal with all that.

Upon the discovery of an egg sac, the Lead Researcher should be notified along with Site Security.

The "along with" bit seems weird in the sentence. Just "the Lead Researcher and Site Security are to be notified" works.

Security personnel are to remove the egg sacs while the spiders are sleeping.

How can anyone tell if spiders are sleeping? Why not just eliminate the risk by spraying the area with some sort of spider-anesthetic or similar slowing agent?

The egg sacs are to be incinerated as soon as possible if not requested for research purposes.

Why not just have a certain number be kept in cold storage, and incinerate the rest if said storage is currently within limit? That way, you always have some on hand for research, and people don't need to ask "hey, did anyone need this thing?" before roasting it.

Just SCP-XXXX is fine. If you have weird stuff like this randomly in a draft, it's going to throw a reviewer off. If the deviation is neither significant to the piece or relevant at all, you don't need it.

is an arachnid

Earlier you say "instances of SCP-XXXX". Is it a single arachnid, or a population of a certain arachnid species?

that resembles members of Loxosceles reclusa, or the Brown Recluse Spider.

I think it's more professional to put Latin name (Common name).

Hunting patterns are similar to Brown Recluse Spiders,

Two issues here. First off, hunting patterns of what? Secondly, it'd be "similar to those of recluse spiders. You're comparing hunting patterns to hunting patterns, not hunting patterns to spiders.

with instances of SCP-XXXX hunting at night.

Use of "with" reads idiomatically incorrect. It's okay in casual language, but SCP articles are meant to sound like professional scientific documents. Try "Instances of SCP-XXXX are nocturnal, and exhibit hunting patterns similar to those of non-anomalous Brown Recluse spiders."

However, instances of SCP-XXXX hunt in groups of 5 or more, and target or humans.

Either you're missing a word, or you've got an extra "or". Also, write out all numbers less than 20.

SCP-XXXX is approximately 10 times larger than the average member of the species,

This doesn't read well, since the species of SCP-XXXX is inherently different from the Brown Recluse species. So this should read "SCP-XXXX individuals are approximately ten times larger than the average member of the Latin name species.

with measured sizes up to 21 cm.

It might be better to give a range for this, since you're missing a tail end of the size variation.

A notable difference between the two is the coloring of SCP-XXXX. SCP-XXXX is completely black, with no noticeable markings.

…so how is it similar to the Brown Recluse? The most widely-known identifying feature of the brown recluse (so far as I know) is the violin-shaped pattern on the cephalothorax. Without that, a naive observer seeing the thing for the first time might draw comparisons with any number of spiders. How did the Foundation manage to ID this thing's closest non-anomalous relative as something that doesn't even share the same coloring?

You might want to establish the appearance first before you distract the audience with the random hunting behavior mention.

The color is uniform throughout SCP-XXXX, and has not been observed to have variation between different instances.

I feel like you don't really need this statement, since it's confirming something that the audience probably wouldn't have questioned. You don't need to say what the SCP doesn't have.

SCP-XXXX's main anomalous feature

"Primary anomalous effect" might be better.

is its capacity to mimic sounds that are associated with humans.

There are birds and occasionally cats who can do this. Also there's a bug that can sound like a chainsaw. And a toad that makes squeaky sounds.

Instances of SCP-XXXX have been recorded to mimic sounds of footsteps, snoring, slamming doors, vibrating phones, dropped glassware, and coughing.

SCP-XXXX will utilize these sounds to make people investigate and isolate themselves.

The structure of the sentence reads reaaaaally sketchily. Parallel structure dictates that it be read "make people investigate (themselves) and isolate themselves". Also, investigate what? And how does it make people isolate themselves by making noises? What prevents people from turning to a friend and saying, "hey, let's go find the source of that weird noise together"?

Following this, SCP-XXXX will ambush its prey. Typically, when ambushing prey, SCP-XXXX will lie in wait on ceilings or in shadow, then will bite its prey.

How did the Foundation figure this out?

The venom of SCP-XXXX is highly necrotic,

D&D uses "necrotic" to describe damage. Venom being "necrotic" doesn't read as smoothly because "necrosis" is "the death of most or all of the cells in an organ or tissue due to disease, injury, or failure of the blood supply", and it's not the venom itself that is dying, but that's what the venom causes.

with no known cure.

Boring. If you make this thing SUPER POWERFUL NOTHING CAN STOP IT YOU WILL DIE, there's no story. Without conflict, you have no adventure.

The dosage of venom given in a single bite is always fatal, with tissue death occurring in as little as 3 hours after the bite.

I think "as few as" is the better wording. Also, did people seriously not keep under close watch whoever got bitten by the thing? Did they just wait around for them to show serious symptoms?

The tissue will become gangrenous, with entire limbs being observed to undergo tissue death.

Why didn't they amputate or attempt something to contain the venom?

Upon the dissection of SCP-XXXX, it was found that the abdominal section of SCP-XXXX contained an air sac connected to a highly evolved set of vocal chords, which led to a "mouth" where SCP-XXXX's sounds are produced. This system replaced a section of heart and intestines, as well as the silk glands as compared to normal Loxosceles Reclusa.

…so why does this thing even attack people? Obviously it doesn't need to digest anything, since no intestines, so… why is it so aggressive? And why people? Why is it always people attacked?

Overall… this is pretty boring. It's effectively just a generic monster (maybe read over this guide to get an idea of what elements to avoid putting in an SCP article) and we've already got a lot of mean ripping stabby bitey monsters on the site.

Also, there's a lot of info here that seems… unusually specific. Keep in mind that as the author, you know the entire story, but the Foundation needs to have discovered what it knows about the SCP through observation and experimentation. How would someone with no prior knowledge whatsoever of the object, using only experimentation and observation, figure out all the facts that are present in the article?

I dunno. We already have a couple things that mimic human voices as a predatory prey lure (my own SCP-1355, for example) and plenty of spiders.

If you really want to make this work, consider a different approach. Why are these spiders the way they are? Are they meant to be this way? How do they feel about it? What are their motivations, their goals for existing? How can the reader relate to this, emotionally or intellectually?